1. Field of the Invention
The present invention relates to a ceramic heater, especially a ceramic heater suitably employed as a glow plug in a diesel engine.
2. Description of the Prior Art
In diesel engines, glow plugs are employed for starting engines in an atmosphere of low temperatures and these glow plugs are required to be provided with a heater having a rapidly heating property in order to improve the startability of engines.
The present inventors have developed a ceramic heater comprising a supporting member made of electrically insulating ceramic sintered body, a heater element formed by bonding a ceramic sintered body of MoSi.sub.2 and Si.sub.3 N.sub.4 to a top end of the supporting member, and lead wires embedded in the supporting member and connected to the heater element (U.S. application Ser. Nos. 717,875 and 739,474).
This ceramic heater has oxidization resistance at high temperatures due to the existence of MoSi.sub.2 and thermal shock resistance due to the existence of Si.sub.3 N.sub.4. Therefore, this ceramic heater can be employed in a combustion chamber of a diesel engine without being covered with any covering member and accordingly, exhibits an excellent rapidly heating property.
The temperature of glow plugs is controlled in accordance with the driving condition of engines. In cold districts where the ambient temperature is not more than -15.degree. C., it takes time until the diesel engines become the state of good startability. So, the diesel engines are earnestly desired to have good startability at low temperatures similar to gasoline engines, and accordingly, the heaters are desired to generate heat of about 1300.degree. C. to 1400.degree. C. for the above purpose.
However, the ceramic heater made of MoSi.sub.2 and Si.sub.3 N.sub.4 cannot maintain good durability at a heating temperature of the heater as high as 1300.degree. C. or more.